1. Field of the Invention
The present invention relates to an improved liquid crystal display device and more particularly, to a liquid crystal display device including an improved crossover portion whose structure prevents disconnections in signal lines.
2. Description of Related Art
Generally, a liquid crystal display (LCD) device includes a bottom plate having a plurality of thin film transistors (TFTs) and a plurality of pixel electrodes disposed thereon, a top plate composed of common electrodes and a color filter for displaying color, a liquid crystal (LC) layer disposed between the top and bottom plates, and a pair of polarizers located on the top and bottom surfaces of the device for selectively transmitting light.
As shown in FIG. 1, a conventional liquid crystal display device comprises a bottom plate containing a thin film transistor (TFT) and a pixel electrode. The bottom plate includes a plurality of gate lines 1 arranged to transmit data signals to the pixel electrode through the TFT and a plurality of data lines 2 crossing the gate lines 1 perpendicularly. As a result, a plurality of spaces are formed between the gate lines 1 and data lines 2. Accordingly, each space formed between the two gate lines and two data lines becomes a pixel region. Each pixel region forms a TFT with the gate line functioning as a gate electrode, the data line functioning as a source electrode, and a drain electrode connected to a pixel electrode.
As shown in FIGS. 2 and 3, the gate line 1 is first formed on the insulator substrate for driving the TFT, the data line 2 is formed thereon perpendicular to the gate line 1 for supplying the data signal to the TFT, and the pixel electrode 4 is formed in the pixel region. As a result, the gate line 1 and data line 2 are provided with a crossover portion A where the data line 1 crosses over the gate line 1.
The liquid crystal display device of the conventional art is prepared by the following process. First, a metal material, such as Al, is deposited to form a first metal layer. Then the first metal layer is selectively patterned using photolithography and etching to form the gate line 1. On the top surface of the formed substrate, a gate insulator layer (not shown) is formed and a semiconductor layer 5 is formed on the active region of the TFT. Then a second metal layer having a metal material, such as Cr, is deposited on the top surface of the formed substrate, and the data line 2 and the drain electrode 3 are formed by selectively removing the second metal layer using photolithography and etching. Finally, the pixel electrode 4 is formed in the pixel region so as to provide connection to the drain electrode 3.
However, such conventional liquid crystal display devices as shown in FIGS. 1 to 3 suffer from a number of problems. For example, since the gate lines and data lines are composed of metal and are formed on a glass or quartz substrate, data or signal lines receive stress due to defects in the metal material and its structure, and due to differences in thermal expansion coefficient. This stress is prominent in the direction that the data lines extend. As a result, the data lines can be easily severed or disconnected.
Additionally, when a metal layer is deposited on the substrate, the deposited layer is selectively patterned to form the data line using photolithography and etching. As a result, the data line crosses over the gate line and step coverage is formed at the crossover portion, i.e. a difference in the position of the data line is formed where the data line crosses over an edge of the gate line. However, due to the step coverage, less metal material is deposited at the crossover portion, especially along the edges of the gate line. Consequently, etchant (etching liquid) seeps into the data line along the two edges of the gate line at the crossover portion.
Accordingly, as shown in FIG. 3, the etchant can seep into the data line 2 as much as the width of the data line 2, causing severance as shown in B. If this occurs, then the data line 12 will be severed. But, even if only the small amount of etchant is seeped, due to line stress in the direction of the data line the severance of the data line may result.